Great Ideas in Science: Lecture 7

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Great Ideas in ScienceLecture 7 Nuclear
Reactions

• Professor Robert Hazen
• UNIV 301 October 11, 2006

Great Idea Nuclear energy depends on the
conversion of mass into energy
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Nuclear Reactions

• Key Idea Nuclear reactions result from the
  rearrangement of an atoms protons and neutrons
  (i.e. the nucleus)
• Key Words
• Proton
• Neutron
• Nucleus
• Isotope
• Radioactivity
• Nuclear Fission
• Nuclear Fusion

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The Building Blocks of Matter

• Of what is matter made?
• Atoms
• Nuclei and electrons
• Quarks

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Key Words About Atoms

• Atom Any object with a nucleus and electrons
• Element An atom with a known number of protons
  (the atomic number)
• Ion An electronically-charged atom with a
  different number of protons () and electrons (-)
• Isotope An element with a known number of
  neutrons

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The Structure of the Atom

• Electrons in shells (energy levels)
• Negatively charged
• Shift during chemical reactions
• Central dense nucleus
• Composed of protons and neutrons
• Positively charged
• Nucleus - Stays put during chemical reactions

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Isotopes Hydrogen Carbon

• H-1 1 proton
• H-2 1 p 1 neutron (Deuterium)
• H-3 1 p 2 n (Tritium)
• C-12 6p 6n
• C-13 6p 7n
• C-14 6p 8n (radioactive isotope)
• For any given element the number of protons is
  fixed

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Chart of the Isotopes (p vs. N)
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Four Fates of Isotopes

• An isotope may be stable
• An isotope may be radioactive
• An isotope may be split apart by fission
• An isotope may combine with another by fusion

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Stable Isotopes

• 99.999 of all the atoms around us
• Examples are carbon-12 and carbon-13
• Different isotopes dont affect chemical
  reactions.
• Used in scientific research to track chemical
  reactions (2 ways)
• As tracers
• Fractionation

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Radioactivity or Radioactive Decay (three kinds)
The spontaneous emission of an energetic particle
by a nucleus

• Alpha radiation
• Beta radiation
• Gamma radiation

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Most Isotopes are Radioactive
STABLE RADIOACTIVE
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Alpha Radiation

• Atom spontaneously loses 2 protons and 2 neutrons
  ( a Helium-4 nucleus)
• Uranium-238 ? Thorium-234 2n 2p

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Beta Radiation

• One neutron spontaneously becomes a proton plus
  an electron
• Thorium-234 ? Proactinium-234

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Gamma Radiation

• Atom spontaneously emits a gamma ray
  (electromagnetic radiation)
• Uranium-238 ? Uranium-238 ?

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SUMMARY The Three Kinds of Radioactive Decay

• Alpha Decay
• Release of a particle with 2 protons and 2
  neutrons
• Beta Decay
• Neutron becomes a proton
• Emission of electron (ß-ray)
• Gamma Radiation
• Electromagnetic radiation

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Radioactivity and Health

• Ionization
• Stripping off electrons
• Long-term effects
• Cancer
• Birth defects

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Half-Life

• The average time for decay of ½ batch of
  radioactive isotopes
• Wide range of half-lives

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Radiometric Dating

• Radiometric dating
• Know half-life of isotope
• Know how much was there
• Measure whats left
• Carbon-14
• Half-life 5700 years

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Radiometric Dating

• Geology
• Need longer half-lives
• Uranium, potassium

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Radioactive Decay Chain (radon)
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Nuclear Fission (Splitting)

• Fission Splitting of nucleus
• A nuclear reactor converts mass to energy

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Nuclear Fission (Splitting)
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Nuclear Fission The Atom Bomb
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Yucca Mountain, Nevada (NIMBY)
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Nuclear Fusion (Fusing)

• Hydrogen atoms combine to form helium
• Some mass is converted into energy

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Nuclear Fusion Hydrogen Bomb
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Stars are Giant Fusion Reactors
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Fates of Stars
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Benefits of Isotopes

• Stable Isotopes
• Medical Research
• Environmental Tracers
• Radioactive Isotopes
• Medical diagnosis
• Cancer treatments
• Environmental tracers
• Age Determination
• Nuclear fission
• Power generation
• Nuclear Fusion
• The Sun